1. Field of the Invention
The invention relates to a method for operating an air conditioner in a motor vehicle, and to an air conditioner for a motor vehicle having a refrigerant circuit.
2. Description of Related Art
EP 0 288 658 B1 describes a method for controlling a motor vehicle air conditioner having a refrigerant circuit. The refrigerant circuit includes at least one output-controlled refrigerant compressor, a refrigerant condenser, an evaporator and a throttling device for the refrigerant, which is arranged upstream of the evaporator. The condenser is assigned a fan for applying air, and a blower is provided for generating an air flow through the evaporator. An electronic control circuit is fed signals from a plurality of sensors, and, taking these signals into account, output signals are calculated in the control circuit which are fed to actuators or drive elements which can be driven electrically for the purpose of influencing the compressor output, the condenser output and the evaporator output.
Refrigerant circuits in motor vehicles are continuously subjected to changing boundary conditions such as, for example, engine driving speed, cooling conditions at the condenser, loading of the evaporator on the air side and refrigerant side, and the required refrigerating performance. Of these changing conditions, the engine driving speeds, which extend over a large range between the minimum speed and the maximum speed of the internal combustion engine, represent a substantial problem for the compressor, the more so since the speed almost always is determined by the power requirements for the vehicle propulsion and not by the power requirements of the air conditioner or of the refrigerant compressor. To make it possible to provide an adequate refrigerating performance for air conditioning the passenger compartment in a motor vehicle even when the internal combustion engine is at idling speed, it is necessary to have high-volume compressors, but these are substantially over-dimensioned for the most frequent operating requirements. For this reason, motor vehicle air conditioners use output-controlled compressors in which the geometrical volumetric capacity can be varied in order to influence the compressor output. Depending on compressor type, the volumetric capacity can be varied by directly changing the stroke or controlling the closing instant of the compression chamber.
It is possible to act in various ways on components of a refrigerant circuit in order to influence its performance. For example, it is known from EP 0 520 628 A2 to reduce the condenser performance when establishing an overcompression, and to increase the condenser performance in the case of an undercompression.